The major goal of this project is to determine how nucleotide metabolism is regulated at both the enzyme and cellular level. Specific studies range from detailed kinetic, physical and regulatory enzyme investigations to the dependence of certain cell types on dietary sources of preformed purine and pyrimidine bases. Three enzymes involved in critical steps in biosynthesis, degradation and interconversion of nucleotides will be studied. The first is adenylosuccinate synthetase which catalyzes the first committed step in AMP formation from IMP. Detailed physical, mechanistic and regulatory investigations will be done. Physical and mechanistic studies will also be done with adenosine deaminase, which catalyzes the irreversible deamination of adenosine or 2'-deoxyadenosine. Site specific mutations will be done in a third study with an unusual form of dihydrofolate reductase, an enzyme involved in deoxynucleotide metabolism. This enzyme which has been isolated from an E. coli plasmid is a small, stable protein which will provide an excellent model system for the study of the role of specific amino acid residues in catalysis and binding. In a related, ongoing study, the requirements of T lymphocytes for a dietary source of preformed purine or pyrimidine bases will be determined. Evaluation of the biochemical basis of this effect will involve analysis of enzyme and metabolite changes as correlated with immune function under different dietary conditions. The combined effect of antimetabolites and diet on the immune system and on lymphoid derived tumors will be determined. These studies should afford new information concerning function and regulation of enzymes involved in nucleotide metabolism and potential new therapeutic protocols for transplant recipients, tumor patients and other patients receiving parenteral or enteral nutritional support.